The ability of ruminants to convert grass and other plant materials, being indigestible to man, into valuable nutritious matter, such as milk and meat, has for thousands of years been essential for the existence of a great many people all over the world.
Man has made butter and cheese from milk for many years. Originally cheese-making served as a method for preserving excess milk in the summer.
The industrial manufacturing of cheese started up more than 100 years ago, as the capability of handling large amounts of milk reached a sufficient technical stage and rennet became commercially available.
General Principle of Cheese-making
In the production of cheese it is necessary to coagulate the cheesemilk to be able to separate the cheese matter e.g. casein from the whey. The milk can be coagulated either by acid treatment or enzyme curdling. In both cases the casein is made insoluble, which leads to the formation of a coherent substances. However, the two coherent coagulated substances (i.e. the cheese material) differ. The coagulate becomes firmer and more elastic when using milk coagulating enzymes, such as rennet (a preparation of rennin which is also called chymosin), in comparison with the coagulate formed when using acids.
Products containing chymosin, which can be isolated from the fourth stomach of calves, have for many years been used for this purpose. Shortage of calf stomachs has in the last decades resulted in the introduction of other milk coagulating enzymes such as bovine pepsin, porcine pepsin as well as microbial enzymes. All these enzymes are characterized by having specificity towards the peptide bond between residue 105 phenylalanine and residue 106 methionine or a bond adjacent to that in kappa-casein. This means that by employing these enzymes in cheese-making, the kappa-casein is split at the junction between para-kappa-casein and a macropeptide moiety called glycomacropeptide (GMP) carrying the negative charges.
When this occurs the macropeptide diffuses into the whey, its stabilizing effect is lost, and the protein micelles can start to aggregate once sufficient amounts of kappa-casein has been hydrolyzed.
This diffusion of macropeptide into the whey means that quite a lot of the milk dry matter is lost into the whey. Whey typically consists of 0.85 % protein, 0.36 % fat and 5.14 % sugars (USDA Table of Standard Reference. U.S. Government Printing Office, 1986). For further elaboration on the enzymatic coagulation of milk see e.g. D. G. Dalgleish in Advanced Dairy Chemistry, Vol 1, ed by P. F. Fox Elsevier, London 1992.
Enzymes and Cheese-making
Enzymes have an important role in the making of cheese and for the flavor, texture, mouth feeling etc. of cheese products.
Certain enzymes are added e.g. to obtain a desired texture. Other enzymes, which are present in the milk raw material, are inactivated e.g. to avoid an undesired flavor.
For instance, the above mentioned milk coagulating enzymes (e.g. rennet) are added at a certain point of time in the cheese-making process to secure the coagulation of the cheese material. Such enzymes are proteases which besides coagulating the milk material further cause a limited degree of breakdown of the milk proteins. This is of importance for the texture and flavor.
Lipases present in the milk will result in liberation of short chain fatty acids from the milk fat triglycerides, which will contribute to a harsh flavor. This can be avoided by heat treating (i.e. usually by pasteurization) the cheesemilk. However, for cheese types such as Danablue and Boerenkaas a harsh flavor is desired.
Lysozyme, which is a glucosidase, is known to be added to the cheese as a preservative. Lysozyme hydrolyzes certain mucopolysaccharides and mucopeptides, which cause lysis of the cell wall of bacteria, such as bacterial cell wall of the genus Clostridium sp.
Transglutaminases are also known to be used for manufacturing milk and milk-like products, such as cheese. Even though the precise action of transglutaminase is not completely understood, it is believed that transglutaminase cross-links proteins in the milk or milk-like product, whereby a lattice or network is generated. This causes gelling of the aqueous phase of such products which can be advantageous.
WO 94/21130 (Novo Nordisk A/S) discloses a method for production of a non- acidified edible gel on milk basis comprising addition of transglutaminase and rennet to milk, followed by a heat treatment. Hereby a functionally and/or organoleptically satisfactory edible gel is obtained, which can be used as mousse, cheese or pudding etc.
From WO 94/21129 (Novo Nordisk A/S) it is known that addition of transglutaminase to milk or milk-like products such as cheese, will lead to products having a pleasant consistency and mouth feeling and exhibit satisfactory organoleptic properties. Further such products can be produced without adding emulsifiers and stabilizers.
WO 93/22930 (Novo Nordisk A/S) describes a method for the production of a milk-like product by adding transglutaminase to a liquid containing milk proteins. The liquid contains Ca.sup.++, if required, in an amount sufficient for the reaction catalyzed by the transglutaminase, and the pH of the liquid, if required, is adjusted to a value of between 5.5 and 7.5. Hereby a milk-like product which is physically more stable is obtained.
From JP-A-5959151 it appears that modified milk products in gel form can be obtained by addition of transglutaminase to milk.
From JP-A-2276541 it appears that a fibrous, tissue containing protein food can be obtained on the basis of a casein solution, transglutaminase and a milk coagulating enzyme.
JP-A-6030770 concerns a method for protein gelation in a solution or slurry containing 1.0 wt. % or more protein by the action of a new transglutaminase isolated from sea squirt (Halocynthia roretzi). It is also mentioned that said transglutaminase can be used for gelling foods such as cheese.
Comments to Prior Art
In conventional processes for making cheese it is a drawback that a considerable amount of protein is lost into the whey, as protein is a valuable component in the cheese product. None of the above mentioned prior art documents solve this problem, as the aim of the preparation methods disclosed in said documents is to stabilize and emulsify milk or milk-like products resulting in a physically improved product. When using one of these disclosed techniques for producing cheese a considerable amount of protein will be lost into the whey, when the whey is separated from the cheese material.
Therefore, it would be desirable to be able to maintain at least some of this above mentioned protein, lost into the whey, in the cheese material when making cheese.